Insulated Gate Bipolar Transistor (IGBT) is an integrated combination of a bipolar transistor and a Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET). The IGBTs have superior on-state characteristics and excellent safe-operating window. The IGBTs in integrated circuits are commonly configured as Lateral Insulated Gate Bipolar Transistors (LIGBTs) and fabricated using a planar process sequence to minimize the cost and the complexity of the integrated circuit manufacturing processes.
A conventional LIGBT includes an n-type region, in which a p-body is formed. A gate is formed over the n-type region. A p+ cathode (which is also a source) is formed in the p-body. A p+ anode (which is also a drain) is formed in the n-type region and on the opposite side of the gate than the p+ cathode. A deep p-well region is formed under the n-type region, and a p-type substrate is under the deep p-well region. The deep p-well region acts as the isolation of the LIGBT. This LIGBT, however, suffers from hole overflow. Since the deep p-well region is formed by implanting p-type impurities deeply into the substrate, the impurity concentration of the deep p-well region is not high enough, and hence the isolation effect is not satisfactory. As a result, the holes in the n-type region overflow across the deep p-well region, and into the p-type substrate. The overflowed holes results in substrate noise and latch-up.
Other LIGBTs may be formed on a silicon-on-insulator substrate to avoid the hole overflow. The n-type region of the respective LIGBT is accordingly in contact with an underlying Buried Oxide (BOX) layer. In these LIGBTs, however, the holes in the n-type region are recombined at the interface between the n-type region and the BOX, causing current gain degradation, the lowering of the device driving capability, and the increase in the leakage current.